1. Field of the Invention
The present invention relates to a spark plug used in an internal combustion engine.
2. Description of the Related Art
In recent years, there have been proposed many variations of a spark plug in which a noble metal chip containing, as a main component, Pt, Ir, or the like is welded to the tip end of an electrode in order to improve spark consumption resistance.
However, in recent years, the temperature within a combustion chamber has tended to become higher with an improvement in the performance of an internal combustion engine. Also, more and more engines have been equipped with a spark plug having a spark discharge portion projecting into the interior of a combustion chamber in order to improve ignition performance. Under these circumstances, the spark discharge portion of a spark plug is exposed to high temperature, with the result that consumption of the noble metal chip of the spark discharge portion proceeds more readily. This tendency is especially significant in spark plugs using an Ir chip, which is susceptible to oxidation/volatilization at high temperature.
The consumption of such a chip at high temperature is considered to occur as follows. The chip undergoes a sputtering-like phenomenon caused by sparks as well as oxidation/corrosion or oxidation/volatilization, so that grain boundaries become fragile, and detachment of grains proceeds due to action of sparks. In this connection, Laid-Open Japanese Patent Applications Nos. 8-37082 and 8-45643 propose methods in which the microstructure of the noble metal chip is controlled such that flat crystal grains are layered in a direction perpendicular to a spark discharge surface, thereby increasing the length of the path along which corrosion of grain boundaries proceeds. As a result, the detachment of the grains of chip is suppressed. However, corrosion and grain detachment cannot be effectively prevented through mere control of the crystal grain morphology of a noble metal chip.